SWI-Prolog offers several ways to store data in globally accessible
memory, i.e., outside the Prolog stacks. Data stored this way
notably does not change on backtracking. Typically it is a bad
idea to use any of the predicates in this section for realising global
variables that can be assigned to. Typically, first consider
representing data processed by your program as terms passed around as
predicate arguments. If you need to reason over multiple solutions to a
goal, consider findall/3, aggregate/3
and related predicates.
Nevertheless, there are scenarios where storing data outside the
Prolog stacks is a good option. Below are the main options for storing
data:
- Using dynamic predicates
- Dynamic predicates are predicates for which the list of clauses is
modified at runtime using asserta/1, assertz/1, retract/1
or
retractall/1.
Following the ISO standard, predicates that are modified this way need
to be declared using the dynamic/1 directive.
These facilities are defined by the ISO standard and widely supported.
The mechanism is often considered slow in the literature. Performance
depends on the Prolog implementation. In SWI-Prolog, querying dynamic
predicates has the same performance as static ones. The manipulation
predicates are fast. Using retract/1
or retractall/1
on a predicate registers the predicate as `dirty'. Dirty predicates are
cleaned by
garbage_collect_clauses/0,
which is normally automatically invoked. Some workloads may result in
significant performance reduction due to skipping retracted clauses
and/or clause garbage collection.
Dynamic predicates can be wrapped using library library(persistency)
to maintain a backup of the data on disk. Dynamic predicates come in two
flavours, shared between threads and local to each
thread. The latter version is created using the directive
thread_local/1.
- The recorded database
- The `recorded database' registers a list of terms with a key,
an atom or compound term. The list is managed using recorda/3, recordz/3
and erase/1.
It is queried using recorded/3.
The recorded database is not part of the ISO standard but fairly widely
supported, notably in implementations building on the `Edinburgh
tradition'. There are few reasons to use this database in SWI-Prolog due
to the good performance of dynamic predicates. Advantages are (1) the
handle provides a direct reference to a term, (2) cyclic terms can be
stored and (3) attributes (section
8.1) are preserved. Disadvantages are (1) the terms in a list
associated with a key are not indexed, (2) the poorly specified
immediate update semantics (see section
4.14.5 applies to the recorded database and (3) reduced portability.
- The flag/3
predicate
- The predicate flag/3
associates one simple value (number or atom) with a key (atom, integer
or compound). It is an old SWI-Prolog specific predicate that should be
considered deprecated, although there is no plan to remove it.
- Using global variables
- The predicates b_setval/2
and nb_setval/2
associate a term living on the Prolog stack with a name, either
backtrackable or non-backtrackable. Backtrackable and non-backtrackable
assignment without using a global name can be realised with setarg/3
and nb_setarg/3.
Notably the latter are used to realise aggregation as e.g., aggregate_all/3
performs.
- Tries
- As of version 7.3.21, SWI-Prolog provides tries (prefix trees)
to associate a term variant with a value. Tries have been
introduced to support tabling and are described in section
4.14.4.
- [ISO]abolish(:PredicateIndicator)
- Removes all clauses of a predicate with functor Functor and
arity
Arity from the database. All predicate attributes (dynamic,
multifile, index, etc.) are reset to their defaults. Abolishing an
imported predicate only removes the import link; the predicate will keep
its old definition in its definition module.
According to the ISO standard, abolish/1
can only be applied to dynamic procedures. This is odd, as for dealing
with dynamic procedures there is already retract/1
and retractall/1.
The abolish/1
predicate was introduced in DEC-10 Prolog precisely for dealing with
static procedures. In SWI-Prolog, abolish/1
works on static procedures, unless the Prolog flag iso
is set to true.
It is advised to use retractall/1
for erasing all clauses of a dynamic predicate.
- abolish(+Name,
+Arity)
- Same as
abolish(Name/Arity). The predicate abolish/2
conforms to the Edinburgh standard, while abolish/1
is ISO compliant.
- copy_predicate_clauses(:From,
:To)
- Copy all clauses of predicate From to To. The
predicate
To must be dynamic or undefined. If To is
undefined, it is created as a dynamic predicate holding a copy of the
clauses of
From. If To is a dynamic predicate, the clauses of
From are added (as in assertz/1)
to the clauses of To.
To and From must have the same arity. Acts as if
defined by the program below, but at a much better performance by
avoiding decompilation and compilation.
copy_predicate_clauses(From, To) :-
head(From, MF:FromHead),
head(To, MT:ToHead),
FromHead =.. [_|Args],
ToHead =.. [_|Args],
forall(clause(MF:FromHead, Body),
assertz(MT:ToHead, Body)).
head(From, M:Head) :-
strip_module(From, M, Name/Arity),
functor(Head, Name, Arity).
- redefine_system_predicate(+Head)
- This directive may be used both in module
user and in
normal modules to redefine any system predicate. If the system
definition is redefined in module user, the new definition
is the default definition for all sub-modules. Otherwise the
redefinition is local to the module. The system definition remains in
the module system.
Redefining system predicate facilitates the definition of
compatibility packages. Use in other contexts is discouraged.
- [ISO,nondet]retract(+Term)
- When Term is an atom or a term it is unified with the first
unifying fact or clause in the database. The fact or clause is removed
from the database. The retract/1
predicate respects the logical update view. This implies that retract/1
succeeds for all clauses that match Term when the predicate
was called. The example below illustrates that the first call
to retract/1
succeeds on
bee on backtracking despite the fact that bee
is already retracted.76Example by
Jan Burse.
:- dynamic insect/1.
insect(ant).
insect(bee).
?- ( retract(insect(I)),
writeln(I),
retract(insect(bee)),
fail
; true
).
ant ;
bee.
If multiple threads start a retract on the same predicate at the same
time their notion of the entry generation is adjusted such that
they do not retract the same first clause. This implies that, if
multiple threads use once(retract(Term)), no two threads
will retract the same clause. Note that on backtracking over retract/1,
multiple threads may retract the same clause as both threads respect the
logical update view.
- [ISO,det]retractall(+Head)
- All facts or clauses in the database for which the head
unifies with Head are removed. If Head refers to a
predicate that is not defined, it is implicitly created as a dynamic
predicate. See also dynamic/1.77The
ISO standard only allows using dynamic/1
as a directive.
- [ISO]asserta(+Term)
- [ISO]assertz(+Term)
- [deprecated]assert(+Term)
- Assert a clause (fact or rule) into the database. The predicate
asserta/1
asserts the clause as first clause of the predicate while
assertz/1
assert the clause as last clause. The deprecated assert/1
is equivalent to assertz/1.
If the program space for the target module is limited (see set_module/1), asserta/1
can raise a
resource_error(program_space) exception. The example below
adds two facts and a rule. Note the double parentheses around the rule.
?- assertz(parent('Bob', 'Jane')).
?- assertz(female('Jane')).
?- assertz((mother(Child, Mother) :-
parent(Child, Mother),
female(Mother))).
- asserta(+Term,
-Reference)
- assertz(+Term,
-Reference)
- [deprecated]assert(+Term,
-Reference)
- Equivalent to asserta/1, assertz/1, assert/1,
but in addition unifies
Reference with a handle to the asserted clauses. The handle
can be used to access this clause with clause/3
and erase/1.
- recorda(+Key,
+Term, -Reference)
- Assert Term in the recorded database under key Key.
Key is a small integer (range min_tagged_integer
...max_tagged_integer,
atom or compound term. If the key is a compound term, only the name and
arity define the key.
Reference is unified with an opaque handle to the record (see
erase/1).
- recorda(+Key,
+Term)
- Equivalent to
recorda(Key, Term, _).
- recordz(+Key,
+Term, -Reference)
- Equivalent to recorda/3,
but puts the Term at the tail of the terms recorded under Key.
- recordz(+Key,
+Term)
- Equivalent to
recordz(Key, Term, _).
- recorded(?Key,
?Value, ?Reference)
- True if Value is recorded under Key and has the
given database Reference. If Reference is given,
this predicate is semi-deterministic. Otherwise, it must be considered
non-deterministic. If neither Reference nor Key is
given, the triples are generated as in the code snippet below.78Note
that, without a given Key, some implementations return
triples in the order defined by recorda/2
and recordz/2.
See also current_key/1.
current_key(Key),
recorded(Key, Value, Reference)
- recorded(+Key,
-Value)
- Equivalent to
recorded(Key, Value, _).
- erase(+Reference)
- Erase a record or clause from the database. Reference is a
db-reference returned by recorda/3, recordz/3
or recorded/3, clause/3,
assert/2, asserta/2
or assertz/2.
Fail silently if the referenced object no longer exists. Notably, if
multiple threads attempt to erase the same clause one will succeed and
the others will fail.
- instance(+Reference,
-Term)
- Unify Term with the referenced clause or database record.
Unit clauses are represented as Head :-
true.
The predicate flag/3
is the oldest way to store global non-backtrackable data in SWI-Prolog.
Flags are global and shared by all threads. Their value is limited to
atoms, small (64-bit) integers and floating point numbers. Flags are
thread-safe. The flags described in this section must not be confused
with Prolog flags described in
section 2.11.
- get_flag(+Key,
-Value)
- True when Value is the value currently associated with Key.
If Key does not exist, a new flag with value `0' (zero) is
created.
- set_flag(+Key,
Value)
- Set flag Key to Value. Value must be an atom,
small (64-bit) integer or float.
- flag(+Key, -Old,
+New)
- True when Old is the current value of the flag Key
and the flag has been set to New. New can be an
arithmetic expression. The update is atomic. This predicate can
be used to create a shared global counter as illustrated in the
example below.
next_id(Id) :-
flag(my_id, Id, Id+1).
Tries (also called digital tree, radix tree or
prefix tree maintain a mapping between a variant of a term (see
=@=/2) and a value. They
have been introduced in SWI-Prolog 7.3.21 as part of the implementation
of tabling. The current implementation is rather immature. In
particular, the following limitations currently apply:
- Tries are not thread-safe.
- Tries should not be modified while non-deterministic predicates such
as trie_gen/3
are running on the trie.
- Terms cannot have attributed variables.
- Terms cannot be cyclic. Possibly this will not change
because cyclic terms can only be supported after creating a canonical
form of the term.
We give the definition of these predicates for reference and
debugging tabled predicates. Future versions are likely to get a more
stable and safer implementation. The API to tries should not be
considered stable.
- trie_new(-Trie)
- Create a new trie and unify Trie with a handle to the trie.
The trie handle is a blob. Tries are subject to atom garbage
collection.
- trie_destroy(+Trie)
- Destroy Trie. This removes all nodes from the trie and causes
further access to Trie to raise an existence_error exception.
The handle itself is reclaimed by atom garbage collection.
- [semidet]is_trie(@Trie)
- True when Trie is a trie object. See also current_trie/1.
- [nondet]current_trie(-Trie)
- True if Trie is a currently existing trie. As this enumerates
and then filters all known atoms this predicate is slow and should only
be used for debugging purposes. See also is_trie/1.
- trie_insert(+Trie,
+Key, +Value)
- Insert the term Key into Trie and associate it
with
Value. Value can be any term. If Key-Value
is already part of Trie, the predicates fails
silently. If Key is in Trie associated with a
different value, a
permission_error is raised.
- trie_update(+Trie,
+Key, +Value)
- As trie_insert/3,
but if Key is in Trie, its associated value is updated.
- trie_insert(+Trie,
+Term, +Value, -Handle)
- As trie_insert/3,
returning a handle to the trie node. This predicate is currently unsafe
as Handle is an integer used to encode a pointer. It was used
to implement a pure Prolog version of the
library(tabling)
library.
- trie_delete(+Trie,
+Key, ?Value)
- Delete Key from Trie if the value associated with Key
unifies with Value.
- trie_lookup(+Trie,
+Key, -Value)
- True if the term Key is in Trie and associated
with
Value.
- trie_term(+Handle,
-Term)
- True when Term is a copy of the term associated with Handle.
The result is undefined (including crashes) if Handle is not
a handle returned by trie_insert_new/3
or the node has been removed afterwards.
- [nondet]trie_gen(+Trie,
?Key, -Value)
- True when Key is associated with Value in Trie.
Backtracking retrieves all pairs. Currently scans the entire trie, even
if Key is partly known. Currently unsafe if Trie
is modified while the values are being enumerated.
- [nondet]trie_property(?Trie,
?Property)
- True if Trie exists with Property. Intended for
debugging and statistical purposes. Retrieving some of these properties
visit all nodes of the trie. Defined properties are
- value_count(-Count)
- Number of key-value pairs in the trie.
- node_count(-Count)
- Number of nodes in the trie.
- size(-Bytes)
- Required storage space of the trie.
- hashed(-Count)
- Number of nodes that use a hashed index to its children.
Traditionally,
Prolog systems used the immediate update view: new clauses
became visible to predicates backtracking over dynamic predicates
immediately, and retracted clauses became invisible immediately.
Starting with SWI-Prolog 3.3.0 we adhere to the logical update
view, where backtrackable predicates that enter the definition of a
predicate will not see any changes (either caused by assert/1
or
retract/1)
to the predicate. This view is the ISO standard, the most commonly used
and the most `safe'.79For example,
using the immediate update view, no call to a dynamic predicate is
deterministic. Logical updates are realised by keeping
reference counts on predicates and generation information on
clauses. Each change to the database causes an increment of the
generation of the database. Each goal is tagged with the generation in
which it was started. Each clause is flagged with the generation it was
created in as well as the generation it was erased from. Only clauses
with a `created' ... `erased' interval that encloses the generation of
the current goal are considered visible.
The indexing capabilities of
SWI-Prolog are described in
section 2.18.
Summarizing, SWI-Prolog creates indexes for any applicable argument,
pairs of arguments and indexes on the arguments of compound terms when
applicable. Extended JIT indexing is not widely supported amoung Prolog
implementations. Programs that aim at portability should consider using term_hash/2
and term_hash/4
to design their database such that indexing on constant or functor
(name/arity reference) on the first argument is sufficient. In some
cases, using the predicates below to add one or more additional columns
(arguments) to a database predicate may improve performance. The overall
design of code using these predicates is given below. Note that as term_hash/2
leaves the hash unbound if Term is not ground. This causes
the lookup to be fast if Term is ground and correct (but
slow) otherwise.
:- dynamic
x/2.
assert_x(Term) :-
term_hash(Term, Hash),
assertz(x(Hash, Term)).
x(Term) :-
term_hash(Term, Hash),
x(Hash, Term).
- [det]term_hash(+Term,
-HashKey)
- If Term is a ground term (see ground/1), HashKey
is unified with a positive integer value that may be used as a hash key
to the value. If Term is not ground, the predicate leaves HashKey
an unbound variable. Hash keys are in the range 0 ... 16,777,215,
the maximal integer that can be stored efficiently on both 32 and 64 bit
platforms.
This predicate may be used to build hash tables as well as to exploit
argument indexing to find complex terms more quickly.
The hash key does not rely on temporary information like addresses of
atoms and may be assumed constant over different invocations and
versions of SWI-Prolog.80Last
change: version 5.10.4 Hashes differ between big and little
endian machines. The term_hash/2
predicate is cycle-safe.bugAll
arguments that (indirectly) lead to a cycle have the same hash key.
- [det]term_hash(+Term,
+Depth, +Range, -HashKey)
- As term_hash/2,
but only considers Term to the specified
Depth. The top-level term has depth 1, its arguments have
depth 2, etc. That is, Depth = 0 hashes nothing; Depth
= 1 hashes atomic values or the functor and arity of a compound
term, not its arguments; Depth = 2 also indexes
the immediate arguments, etc.
HashKey is in the range [0 ...Range-1]. Range
must be in the range [1 ... 2147483647]
- [det]variant_sha1(+Term,
-SHA1)
- Compute a SHA1-hash from Term. The hash is represented as a
40-byte hexadecimal atom. Unlike term_hash/2
and friends, this predicate produces a hash key for non-ground terms.
The hash is invariant over variable-renaming (see =@=/2)
and constants over different invocations of Prolog.bugThe
hash depends on word order (big/little-endian) and the wordsize (32/64
bits).
This predicate raises an exception when trying to compute the hash on
a cyclic term or attributed term. Attributed terms are not handled
because subsumes_chk/2
is not considered well defined for attributed terms. Cyclic terms are
not supported because this would require establishing a canonical cycle.
That is, given A=[a|A] and B=[a,a|B],
A and B should produce the same hash. This is not
(yet) implemented.
This hash was developed for lookup of solutions to a goal stored in a
table. By using a cryptographic hash, heuristic algorithms can often
ignore the possibility of hash collisions and thus avoid storing the
goal term itself as well as testing using =@=/2.
- [det]variant_hash(+Term,
-HashKey)
- Similar to variant_sha1/2,
but using a non-cryptographic hash and produces an integer result like term_hash/2.
This version does deal with attributed variables, processing them as
normal variables. This hash is primarily intended to speedup finding
variant terms in a set of terms.
bugAs variant_sha1/2,
cyclic terms result in an exception.